Personally, regarding Thor's scene with Jane when they went into that cave after Loki had "died", I would have had them having a serious conversation about recent events, along with possibly having Thor doubt himself which would lead to Jane helping to reassure him, thus helping build some validity in their relationship within the MCU.
And instead of her lunch date's phone call being the reason that helped Jane figure out that there was a portal there, she could have just simply spotted the keys and make the connection from there.
Also, after Frigga's death...instead of cutting right away to Selvig's scene, I would have had a moment where Thor was taking some time alone to mourn for his mother on his own and seeing first hand how this was affecting his father...we could even insert the Warriors Three (or two) and Sif trying to console him as well about it.
And I would have personally gotten rid of Darcy's BF, place Hawkeye in the film as a SHIELD liaison who comes to London after being alerted about the mysterious portals there and helping Thor take down the dark elves when they arrive on Earth.